When single electricity storage elements are increased in capacity and connected in series in order to realize an increase in the capacity of a electricity storage system, there is a problem in that temperature variation occurs in the series electricity storage elements and power supply to an apparatus connected to the electricity storage system is interrupted when at least one of the electricity storage elements is broken down. Therefore, small-capacity electricity storage elements equivalent to the number of necessary voltage stages are connected in series and a plurality of the electricity storage element rows are connected in parallel. When a part of the electricity storage elements are broken down, only the electricity storage element row to which the broken-down electricity storage elements belong is disconnected to avoid the interruption of the power supply to the apparatus connected to the electricity storage system. By replacing only the broken-down electricity storage element row, it is possible to reduce life cycle costs of the electricity storage system.
However, if a large number of electricity storage elements are connected in series and in parallel and used, because of variation during manufacturing of the electricity storage elements and a difference in an environment of use (e.g., an ambient temperature), an internal short-circuit sometimes occurs only in a part of the electricity storage elements. Consequently, an electric current equal to or larger than an allowable current flows to a part of the electricity storage elements and the electricity storage elements abnormally generate heat and are easily broken down. Therefore, it is anticipated that the electricity storage element row to which the broken-down electricity storage elements belong is disconnected (paralleled off) from the parallel-connected electricity storage system using a switch and replaced with other electricity storage elements. However, in the case of an electricity storage system for the purpose of suppressing power fluctuation, it is necessary to replace electricity storage elements without stopping charging and discharging. Therefore, it is necessary to reconnect (parallel on) the electricity storage element row after the replacement in a state in which the parallel-connected electricity storage system is performing the charging and discharging. However, when paralleling on the electricity storage element row, if there is a voltage difference between the parallel-connected electricity storage system and the electricity storage element row after the replacement, an electric current equal to or larger than the allowable current sometimes flows to a part of the electricity storage elements.
For example, Patent Literature 1 proposes an electricity storage system that switches on only an electricity storage element row having a predetermined voltage difference.